1. Field of the Invention
The present invention relates to a radio frequency amplifying circuit and a power amplifying module including the radio frequency amplifying circuit.
2. Description of the Related Art
A mobile communication device such as a mobile phone uses a radio frequency (RF) amplifying circuit to amplify power of a radio frequency signal to be transmitted to a base station. Such an RF amplifying circuit includes an amplifying circuit that amplifies an RF signal and a bias circuit that biases a transistor of the amplifying circuit. See, for example, Japanese Patent Publication 11-330866, hereinafter referred to as JP-A-11-330866.
FIG. 16 is a diagram showing a common configuration of an amplifying circuit and a bias circuit described in JP-A-11-330866 as a conventional technique. An amplifying circuit 1601 amplifies an RF signal (RFIN) input to a base of the amplifying circuit 1601 and outputs the amplified RF signal (RFOUT). A bias circuit 1602 biases a transistor 1603 forming the amplifying circuit 1601 and has an emitter-follower configuration. In many cases, a battery voltage VBAT is applied to a collector of a transistor 1604 forming the bias circuit 1602.
In such a configuration, when the transistors 1603 and 1604 are, for example, heterojunction bipolar transistors (HBTs), a base-emitter voltage VBE of each transistor is around 1.3 V. Thus, the battery voltage VBAT needs to be about 2.8 V in order to drive the transistor 1604. Thus, the minimum value of the battery voltage VBAT is generally, for example, about 2.9 V.
In recent years, for mobile communication devices such as mobile phones, there has been a demand to reduce the minimum value of the battery voltage VBAT down to, for example, about 2.5 V in order to increase call time and communication time. However, in a configuration using the bias circuit 1602 of the emitter follower type as described above, the battery voltage VBAT needs to be about 2.8 V, precluding the above-described demand from being met.